mfd: Use regmap for the mc13xxx-core register access

This change converts the mc13xxx core to use regmap rather than direct
spi r/w.
The spidev member of mc13xxx struct becomes redundant and is removed.
Extra debugging aids are added to mc13xxx_reg_rmw.
Mutex init is moved to before regmap init.

Signed-off-by: Marc Reilly <marc@cpdesign.com.au>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>